Display screen or portion thereof with a transitional graphical user interface



FIG. 1 is a first image in a sequence for a display screen or portionthereof with a transitional graphical user interface showing our newdesign;

FIG. 2 is a second image thereof;

FIG. 3 is a third image thereof;

FIG. 4 is a fourth image thereof;

FIG. 5 is a fifth image thereof;

FIG. 6 is a sixth image thereof; and,

FIG. 7 is a seventh image thereof.

The broken lines showing a display screen or portion thereof, text,symbols, and portions of the graphical user interface illustrateportions of the article, and form no part of the claimed design.

The appearance of the graphical user interface transitions sequentiallybetween the images shown in FIGS. 1-7. The process or period in whichone image transitions to another forms no part of the claimed design.

CLAIM The ornamental design for a display screen or portion thereof witha transitional graphical user interface, as shown and described.